US Patent 7364659 Preparation of asymmetric polyethylene hollow fiber membrane

A porous polyethylene hollow fiber membrane having a pore-size gradient across the inner and outer surfaces thereof is prepared by introducing, during the cooling step of a melt-spun polyethylene hollow fiber, a nitrogen flow and a solvent having a boiling point in the range of 30 to 80° C. to the inner and outer surfaces of the melt-spun hollow fiber, respectively.